1. Field of the Invention
In general, the present invention relates to the down-sizing of an optical device making use of a magneto-optical effect. In particular, the present invention relates to an optical device such as an optical attenuator using the Faraday rotation generated by a magneto-optical crystal.
2. Description of the Related Art
When a beam passes through a magneto-optical crystal such as YIG (Yttrium Iron Garnet) placed in a magnetic field, a Faraday-rotation angle caused by a magneto-optical effect is provided to the beam in accordance with the magnitude and direction of a magnetization vector of the magneto-optical crystal as well as the thickness of the magneto-optical crystal. An optical device based on this principle is called a Faraday rotator. A Faraday rotator is implemented by applying a magnetic field generated by a permanent magnet to a magneto-optical crystal. Since the magnitude and the direction of a magnetization vector applied to the magneto-optical crystal by the permanent magnet are fixed, the Faraday-rotation angle in the Faraday rotator also does not change as well.
There has been proposed a variable optical attenuator including a Faraday rotator wherein a magnetic field is applied to a magneto-optical crystal by only one electromagnet as described in documents such as Japanese Patent Laid-open No. Hei1-204021. In case that only one electro-magnet is used, however, the magnetization of the magneto-optical crystal is not always saturated. When the magnetization of the magneto-optical crystal is not saturated, a number of magnetic domains are generated inside the magneto-optical crystal. The existence of such numerous magnetic domains causes the reproducibility of the attenuation effect of the optical attenuator to deteriorate and, even if satisfactory reproducibility can be preserved, a smooth change of the attenuation is difficult to produce. In addition, the existence of the numerous magnetic domains also gives rise to an attenuation effect in which light scattering in boundary surfaces among the magnetic domains is difficult to control.
Inventors of the present invention have proposed an optical device wherein the Faraday-rotation angle can be changed with the magnetization of the magneto-optical crystal saturated by the use of a combination of a permanent magnet and an electromagnet. For details, refer to OAA, FD9, pages 154 to 157, 1996 authored by Fukushima et al. This optical device is a variable optical attenuator which has a characteristic wherein, by varying a driving current to values in the range 0 to 40 mA, the attenuation can be changed smoothly within the range 1.6 to 25 dB.
The Faraday rotator with a variable Faraday-rotation angle like the one described above can be applied to devices such as a polarization controller for arbitrarily varying the polarization state and a variable optical attenuator. For example, provided at a practical scale with dimensions of 30 mm.times.25 mm.times.12 mm, the optical attenuator utilizing a permanent magnet and an electro-magnet described above can be incorporated in an optical repeater or other devices as it is. If the use of a number of optical attenuators is taken into consideration, however, more compact and lower-cost devices are in demand.